1. Field of the Invention
The present invention relates to a porous Si3N4 for separating or removing foreign matters from a liquid such as water used to polish a semiconductor, and a producing method thereof.
2. Description of the Background Art
Si3N4 powder or metal silicon powder may be used as a raw material of the Si3N4 sintered body. WO 94/27929 (EP 0 653 392 A1) discloses a Si3N4 porous body with entangled prismatic β-Si3N4 particles having an aspect ratio of at least 3, which is produced by sintering α-Si3N4 powder as the raw material added with a compound of a rare earth element. This porous body is a high-porosity and high-strength porous ceramic having prosity of at least 30% and an average pore diameter in a range of 0.05–12 μm. Such characteristics are achieved by the fact that the Si3N4 crystal grain is in the form of hexagonal column.
Japanese Patent Laying-Open No. 2001-316188 (US 2001/0036531 A1) discloses an invention related to a silicon nitride porous body using metal silicon powder and silicon oxide powder as its starting materials and having a generally columnar outer shape to increase a contact area with a fluid such as a gas. This porous body is produced through the following steps. Firstly, a molding aid and water are added to a mixture of 100 parts by mass of metal silicon particles having an average particle diameter of 1–150 μm and 0.2–45 parts by mass, in terms of SiO2, of silicon oxide particles, followed by kneading to obtain a kneaded product. Next, the kneaded product is extrusion molded by means of a mold to have at least two through holes parallel to each other. The obtained extrusion-molded body is heat treated in a nitrogen atmosphere in two steps, at a temperature of 1200–1400° C. in the first step, and thereafter, at a temperature of 1500–1800° C. in the second step. The porous body thus obtained has a great number of columnar crystals precipitated on the surfaces of the through holes and extending almost perpendicularly toward the inside of the through holes.
When the Si3N4 powder is used as the starting material as in WO 94/27929, however, it is not possible to obtain an inexpensive Si3N4 porous body due to the high raw material cost. In the case of Japanese Patent Laying-Open No. 2001-316188 using metal silicon and silicon dioxide as the starting materials, columnar crystals are precipitated in the sintered body. The mechanism of precipitation of the columnar crystals is considered as follows. A gas phase of silicon monoxide generated in the flow paths of the porous body is reacted with residual carbon and nitrogen to generate silicon nitride particles, which grow with the silicon nitride particles formed on the surfaces of the through holes as nuclei, and form the columnar crystals toward the inside of the through holes. According to an embodiment of Japanese Patent Laying-Open No. 2001-316188, the columnar crystals obtained are very long in a range of 0.1–0.8 mm, which are considered to be fibrous crystals. On the other hand, although not stated explicitly, it is thought that the inside of the sintered body is filled with globular crystals. In other words, since the columnar crystals do not grow inside the sintered body, the sintered body obtained by the invention of Japanese Patent Laying-Open No. 2001-316188 is considered to be low in porosity and poor in strength, and thus, it would be difficult to obtain Si3N4 having uniformly high porosity from the surface to the inside of the sintered body.